This invention relates to a process for the preparation of polyamines which are modified predominantly or exclusively in the polyisocyanate component by the hydrolysis of isocyanate prepolymers.
Several processes for the preparation of polyether polyamines containing aromatically bound amino end groups which polyamines are useful starting components for the polyisocyanate polyaddition processes are known to those in the art. These methods are summarized in the description of the state of the art given in German Offenlegungsschriften Nos. 2,948,419 and 3,039,600. These Offenlegungsschriften are directed to processes for the hydrolysis of isocyanate prepolymers based on polyethers, using alkali metal hydroxides in aqueous media. The polyamine is freed from the carbamate intermediate stage by the addition of acid or of acid ion exchangers.
It is also known that use of polymer modified polyether polyols which correspond to the so called "polymer polyols" results in polyurethanes having improved properties. Polymer modified polyoxyalkylene polyamines obtained by graft polymerization of monomers in polyoxyalkylene polyamines terminated with aliphatic amino groups have been described in U.S. Pat. No. 4,286,074. These aliphatic polyamines are, however, relatively difficult to obtain and are too highly reactive for many purposes.
It has been found that when modified polyether polyamines are prepared either by hydrolyzing an isocyanate prepolymer made with polymer polyols (i.e., relatively high molecular weight polyols) under alkaline conditions or by hydrolyzing isocyanate prepolymers and then modifying the thus-produced (aromatic) polyamines, the products obtained are highly viscous and in many cases can only be worked up using techniques such as intense heating.
A method for obtaining polymer modified polyisocyanates based on diphenyl methane which have a relatively low viscosity has been disclosed in European Patent Application EP No. 0,037,112. These products are polymethylene polyphenyl polyisocyanates and their functional derivatives, e.g. carbodiimide- or urethone imine-modified diphenyl methane diisocyanates and similar compounds.